According to EW, Warner Bros’ first idea for the spin-off was to make a faux documentary about Newt Scamander, the “author” of Fantastic Beasts (a book released in 2001 with proceeds going to charity). But Rowling had a better idea for her first-ever screenplay:

Eccentric magizoologist Newt Scamander (Oscar winner Eddie Redmayne) comes to New York (for a reason we won’t disclose) with his trusty weathered case. This case is one of those way-way-way-bigger-on-the-inside magical devices, and within are expansive habitats for a collection of rare and endangered magical creatures from Newt’s travels around globe. He discovers the American wizarding community is fearfully hiding from Muggles … and the threat of public exposure is an even graver concern than in the UK (remember the Salem witch trials?). Fantastic Beasts is the story of what happens when this uniquely skilled English wizard travels to wiz-phobic America and a variety of his creatures, some quite dangerous … get out of their case.

And that’s just the set up—EW notes that “the movie is primarily about what happens after this.”